1. Field of the Invention
The present invention relates to a charge pump circuit.
2. Description of the Related Art
A conventional charge pump circuit is described. FIG. 3 is a diagram illustrating a three-stage charge pump circuit. The conventional charge pump circuit includes diodes D01 to D04 connected in series, capacitors C01 to C03, an output capacitor C0L, a clock driver 201, and an output load 202.
A power supply voltage VDD is applied as an input voltage to an anode of the diode D01. The clock driver 201 converts supplied clock pulses CLK and CLKX into clock pulses CLK2 and CLKX2 having voltage amplitudes based on the power supply voltage VDD so as to output the converted clock pulses CLK2 and CLKX2. The clock pulses CLK and CLKX and the clock pulses CLK2 and CLKX2 have opposite phases to each other, respectively. The clock pulse CLK2 is supplied to the capacitor C01 and the capacitor C03. The clock pulse CLKX2 is supplied to the capacitor C02. A stepped-up voltage is delivered from a cathode of the diode D04 and is accumulated in the output capacitor C0L.
A voltage step-up operation of the conventional charge pump circuit is described as follows.
The power supply voltage VDD applied to the anode of the diode D01 is dropped by the diode D01 to be a voltage of (VDD−Vf) at a connection point AA. First, the capacitor C01 is charged based on the voltage (VDD−Vf) at the connection point AA when a voltage level of the clock pulse CLK2 is the ground voltage. Next, when the voltage level of the clock pulse CLK2 becomes VDD, the voltage at the connection point AA increases to (2VDD−Vf). On this occasion, the voltage level of the clock pulse CLKX2 is the ground voltage. Therefore, the voltage at the connection point BB becomes the voltage of (2VDD−2Vf) that is a voltage of the voltage (2VDD−Vf) at the connection point AA, which is dropped by the diode D02.
When the clock pulses CLK2 and CLKX2 are inverted, the voltage at the connection point BB becomes (3VDD−2Vf) by the same voltage step-up operation as that described above. Further, this operation is repeated until a voltage at a connection point CC becomes (4VDD−3Vf).
The voltage of (4VDD−3Vf) at this connection point CC is dropped by the diode D04 to become the stepped-up voltage of (4VDD−4Vf) at the output terminal DD.
The power supply voltage VDD is stepped up when a series of voltage step-up operations as described above is repeatedly performed (see Japanese Patent Application Laid-Open No. 2002-233134, for example).
However, since each of the diodes has a temperature characteristic, the forward voltage Vf generated in each of the diodes decreases when the temperature rises while the forward voltage Vf increases when the temperature drops. Therefore, if the temperature varies, the stepped-up voltage (4VDD−4Vf) as the output voltage of the charge pump circuit is also varied.